Periodontitis is an infectious disease resulting in an often-painless destruction of tooth supporting tissues (the periodontium) and enhances the risk for various systemic diseases including atherosclerosis, coronary heart disease, stroke and infants with low birth weight. Active periodontitis occurs in a susceptible host and in the presence of pathogenic species in combination with low concentrations of so-called "beneficial bacteria". The success of periodontal therapy primarily depends upon dealing with the negative environmental/behavioral factors and the reduction/elimination of periodontopathogens in combination with the re-establishment of a more suitable environment (less anaerobic) for a beneficial microbiota. Even after therapy, the presence of pathogenic species in subclinical levels is often encountered so that there remains a continuous threat for further periodontal destruction and disease. Therefore, this project aims to improve treatment outcome by guiding the periodontal pocket recolonization after periodontal therapy by local administration of beneficial bacteria (probiosis). An optimal mixture of known beneficial periodontal bacteria will be determined in vitro by investigating the adhesion of these bacteria to dentine- and enamel surfaces and epithelial cells in relation to their probiotic effect using flow cell microscopy and fluorescence microscopy. The in vivo effects of the therapy will be evaluated using a split mouth study design in a beagle dog model for periodontitis. After lesion induction and establishment of severe periodontitis in 8 beagle dogs (20 lesions/dog), 8 lesions do not receive treatment (reservoir for pathogenic recolonization of the treated lesions), 4 lesions receive mechanical debridement (classic strategy), 4 lesions receive mechanical debridement followed by a single subgingival application of the probiotic mixture (experimental treatment 1) and 4 lesions receive mechanical debridement followed by a repeated subgingival application (week 1, 2 and 4) of the probiotic mixture (experimental treatment 2) in each dog. Plaque samples will be obtained from all pockets after 2, 4, 6, 8, and 12 weeks and evaluated via standard culture techniques and checkerboard DNA-DNA hybridization. Clinical data will be collected after 2, 4, 6, 8 and 12 weeks. The results should indicate the possible role for commensal organisms in periodontal disease and subgingival biofilm formation.